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if                                         TRITUBERCUL/F                                       $7
But this may be only a proof that the mulfcitabercular ante-
dates the tritubercular. It may be., indeed, that the mammalian
tooth was already differentiated among the mammal-like Saurians
and that from such a form as Cynognathus the Eutheria and other
forms in which a tritubercular arrangement can be detected were
evolved, and from such form as Tritylodon the Monotre-
matous branch of the mammals. This way of looking at the
matter harmonises a much - disputed question, but involves
a diphyletic origin of the mammals—an origin which for other
reasons is not without its supporters.
We shall now attempt to give a general idea of the facts and
arguments which support or tend to support " trituberculy." As
a matter of fact the name is inaccurate; for the holders of this
view do not derive the mammalian molar from a trituberculate
condition, but in the first place from a simple cone such as that
of a crocodile!
To this main and at first only cusp came as a reinforcement
an additional cusp at each side, or rather at each end, having
regard to their position with reference to the long axis of the
jaw. This stage is the " triconodoiit " stage, and teeth exist among
living as well as extinct mammals which show this early form of
tooth. We have, indeed, the genus Triconodon, so named on that
very account. Among living mammals the Seals and the Thyla-
cine all show some triconodont teeth. A Toothed Whale, it may
be remarked, is a living example of a mammal with monocono-
dont teeth. The three primary cusps, as the supporters of Cope's
theory of trituberculism denominate them, are termed respectively
the protocone, paracone, and metacone, or, if they are in the teeth of
the lower jaw, protoconid, paraconid, and metaconid. At a slightly
later stage, or coineidently, a rim partly surrounded the crown of
the tooth; the rim is known as the cingulum, and from a pro-
minent elevation of this rim a fourth cusp, the hypocone, was
developed. The three main cones then moved, or rather two of them
moved, so as to form a triangle; this is the tritubercular stage.
Teeth of this pattern are common, and occur in 'such ancient
forms as Tnsectivora and Lemurs, besides numerous extinct groups.
An amendment has been suggested, and that is to term the teeth
with the simple primitive triangle " trigonodont," and to reserve
the term tritubercular for those teeth in which the hypocone has
appeared. The platform bearing the hypocone widened into the